A silicon-on-insulator substrate having improved thickness uniformity as well as a method of fabricating the same is provided. Specifically, improved thickness uniformity of a SOI substrate is obtained in the present invention by subjecting a bonded or SIMOX (separation by ion implantation of oxygen) SOI substrate to a high-temperature oxidation process that is capable of improving the thickness uniformity of said SOI substrate. During this high-temperature oxidation process surface oxidation of the superficial Si-containing (i.e., the Si-containing layer present atop the buried oxide (BOX) region) occurs; and (ii) internal thermal oxidation (ITOX), i.e., diffusion of oxygen via the superficial Si-containing layer into the interface that exists between the BOX and the superficial Si-containing layer also occurs. Uniformity is achieved since regions of the SOI substrate which have thicker Si get less ITOX, but more surface oxidation create a thicker surface oxide (and hence thinner superficial Si-containing layer). However, regions of the SOI substrate which are thinner get more ITOX and have thinner surface oxide (and hence thicker superficial Si-containing). This mechanism continues to self-adjust the SOI substrate thickness with oxidation time to improve the SOI thickness uniformity. It has been observed that the closer the oxidizing surface is to the BOX, the more uniform the superficial Si-containing layer becomes.